35 research outputs found
Theoretical analysis of quantum dynamics in 1D lattices: Wannier-Stark description
This papers presents a formalism describing the dynamics of a quantum
particle in a one-dimensional tilted time-dependent lattice. The description
uses the Wannier-Stark states, which are localized in each site of the lattice
and provides a simple framework leading to fully-analytical developments.
Particular attention is devoted to the case of a time-dependent potential,
which results in a rich variety of quantum coherent dynamics is found.Comment: 8 pages, 6 figures, submitted to PR
Wavepacket reconstruction via local dynamics in a parabolic lattice
We study the dynamics of a wavepacket in a potential formed by the sum of a
periodic lattice and of a parabolic potential. The dynamics of the wavepacket
is essentially a superposition of ``local Bloch oscillations'', whose frequency
is proportional to the local slope of the parabolic potential. We show that the
amplitude and the phase of the Fourier transform of a signal characterizing
this dynamics contains information about the amplitude and the phase of the
wavepacket at a given lattice site. Hence, {\em complete} reconstruction of the
the wavepacket in the real space can be performed from the study of the
dynamics of the system.Comment: 4 pages, 3 figures, RevTex
Angular momentum extraction by gravity waves in the Sun
We review the behavior of the oscillating shear layer produced by gravity
waves below the surface convection zone of the Sun. We show that, under
asymmetric filtering produced by this layer, gravity waves of low spherical
order, which are stochastically excited at the base of the convection zone of
late type stars, can extract angular momentum from their radiative interior.
The time-scale for this momentum extraction in a Sun-like star is of the order
of 10^7 years. The process is particularly efficient in the central region, and
it could produce there a slowly rotating core.Comment: 9 pages, 3 figues, accepted by Astrophysical Journal Letter, 26 June
200
Quantum Chaos of a particle in a square well : Competing Length Scales and Dynamical Localization
The classical and quantum dynamics of a particle trapped in a one-dimensional
infinite square well with a time periodic pulsed field is investigated. This is
a two-parameter non-KAM generalization of the kicked rotor, which can be seen
as the standard map of particles subjected to both smooth and hard potentials.
The virtue of the generalization lies in the introduction of an extra parameter
R which is the ratio of two length scales, namely the well width and the field
wavelength. If R is a non-integer the dynamics is discontinuous and non-KAM. We
have explored the role of R in controlling the localization properties of the
eigenstates. In particular the connection between classical diffusion and
localization is found to generalize reasonably well. In unbounded chaotic
systems such as these, while the nearest neighbour spacing distribution of the
eigenvalues is less sensitive to the nature of the classical dynamics, the
distribution of participation ratios of the eigenstates proves to be a
sensitive measure; in the chaotic regimes the latter being lognormal. We find
that the tails of the well converged localized states are exponentially
localized despite the discontinuous dynamics while the bulk part shows
fluctuations that tend to be closer to Random Matrix Theory predictions. Time
evolving states show considerable R dependence and tuning R to enhance
classical diffusion can lead to significantly larger quantum diffusion for the
same field strengths, an effect that is potentially observable in present day
experiments.Comment: 29 pages (including 14 figures). Better quality of Figs. 1,3 & 9 can
be obtained from author
Reflection and Ducting of Gravity Waves Inside the Sun
Internal gravity waves excited by overshoot at the bottom of the convection
zone can be influenced by rotation and by the strong toroidal magnetic field
that is likely to be present in the solar tachocline. Using a simple Cartesian
model, we show how waves with a vertical component of propagation can be
reflected when traveling through a layer containing a horizontal magnetic field
with a strength that varies with depth. This interaction can prevent a portion
of the downward-traveling wave energy flux from reaching the deep solar
interior. If a highly reflecting magnetized layer is located some distance
below the convection zone base, a duct or wave guide can be set up, wherein
vertical propagation is restricted by successive reflections at the upper and
lower boundaries. The presence of both upward- and downward-traveling
disturbances inside the duct leads to the existence of a set of horizontally
propagating modes that have significantly enhanced amplitudes. We point out
that the helical structure of these waves makes them capable of generating an
alpha-effect, and briefly consider the possibility that propagation in a shear
of sufficient strength could lead to instability, the result of wave growth due
to over-reflection.Comment: 23 pages, 5 figures. Accepted for publication in Solar Physic
The evolution of rotating stars
First, we review the main physical effects to be considered in the building
of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The
internal rotation law evolves as a result of contraction and expansion,
meridional circulation, diffusion processes and mass loss. In turn,
differential rotation and mixing exert a feedback on circulation and diffusion,
so that a consistent treatment is necessary.
We review recent results on the evolution of internal rotation and the
surface rotational velocities for stars on the Upper MS, for red giants,
supergiants and W-R stars. A fast rotation is enhancing the mass loss by
stellar winds and reciprocally high mass loss is removing a lot of angular
momentum. The problem of the ``break-up'' or -limit is critically
examined in connection with the origin of Be and LBV stars. The effects of
rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the
blue to red supergiant ratios, the formation of W-R stars, the chemical
abundances in massive stars as well as in red giants and AGB stars, are
reviewed in relation to recent observations for stars in the Galaxy and
Magellanic Clouds. The effects of rotation on the final stages and on the
chemical yields are examined, as well as the constraints placed by the periods
of pulsars. On the whole, this review points out that stellar evolution is not
only a function of mass M and metallicity Z, but of angular velocity
as well.Comment: 78 pages, 7 figures, review for Annual Review of Astronomy and
Astrophysics, vol. 38 (2000
Immunomagnetic microbeads for screening with flow cytometry and identification with nano-liquid chromatography mass spectrometry of ochratoxins in wheat and cereal
Multi-analyte binding assays for rapid screening of food contaminants require mass spectrometric identification of compound(s) in suspect samples. An optimal combination is obtained when the same bioreagents are used in both methods; moreover, miniaturisation is important because of the high costs of bioreagents. A concept is demonstrated using superparamagnetic microbeads coated with monoclonal antibodies (Mabs) in a novel direct inhibition flow cytometric immunoassay (FCIA) plus immunoaffinity isolation prior to identification by nano-liquid chromatographyâquadrupole time-of-flight-mass spectrometry (nano-LC-Q-ToF-MS). As a model system, the mycotoxin ochratoxin A (OTA) and cross-reacting mycotoxin analogues were analysed in wheat and cereal samples, after a simple extraction, using the FCIA with anti-OTA Mabs. The limit of detection for OTA was 0.15Â ng/g, which is far below the lowest maximum level of 3Â ng/g established by the European Union. In the immunomagnetic isolation method, a 350-times-higher amount of beads was used to trap ochratoxins from sample extracts. Following a wash step, bound ochratoxins were dissociated from the Mabs using a small volume of acidified acetonitrile/water (2/8âv/v) prior to separation plus identification with nano-LC-Q-ToF-MS. In screened suspect naturally contaminated samples, OTA and its non-chlorinated analogue ochratoxin B were successfully identified by full scan accurate mass spectrometry as a proof of concept for identification of unknown but cross-reacting emerging mycotoxins. Due to the miniaturisation and bioaffinity isolation, this concept might be applicable for the use of other and more expensive bioreagents such as transport proteins and receptors for screening and identification of known and unknown (or masked) emerging food contaminants
New
We report here the discovery of very low amplitude oscillations in
the Pleiades star HD 23628. The 14-nights high quality light curve
designates HD 23628 as a new member of the ÎŽ Scuti stars
class. Amplitude spectra indicate a multi-periodic pulsation
behaviour, and four pulsation frequencies have been detected so
far. The frequency distribution suggests the presence of nonradial
modes. Pulsation constant values of the four modes are distributed
in the range corresponding to fundamental f to p4 radial modes